A search circuit for achieving synchronization in spread spectrum communications is constructed as set forth below. For example, in a case where a correlator is used for despreading, the correlator multiplies an input signal by a spreading code prepared internally of the search circuit and is capable of obtaining the signal that prevailed prior to spreading by integration over a length of time substantially the same as the duration of the unit signal prior to spreading. In a case where the spreading code prepared within the search circuit is not synchronized to the spreading code of the transmitting party, the integrated output becomes almost zero.
Accordingly, until the desired signal level is obtained by shifting the phase of a spread signal oscillator in the search circuit a fixed amount, this repetitive operation is repeated to achieve synchronization of the spread signals. A circuit which performs synchronization of spread signals by such method is referred to as a "sliding correlator".